The present invention relates to a bipolar transistor circuit. More particularly, the invention relates to a bipolar transistor amplifier circuit of high efficiency and high output, suitable for use in a microwave band.
In the conventional microwave high output amplifier circuit, NPN type bipolar transistor has been typically employed as an amplifying oscillation element. Such NPN type bipolar transistor is formed primarily of silicon (Si) as a primary composition. The P type base region is situated in highly doped condition in order to lower a base resistance, and the emitter is in highly doped condition.
In such bipolar transistor, the base is inherently selected to be in the lower doped condition than the emitter to result in low reverse withstanding voltage between the base and the emitter in the order of 3 volts. Namely, the doping amount at the base is required to be lower than that of the emitter to certainly provide sufficient hfe. Therefore, upon application of reverse voltage, the transition region of P-N junction is spread toward the base side so that the reverse withstanding voltage becomes inversely proportional to the doped amount at the base side.
The relationship between unbalance brake voltage and doped amount in the P-N junction has been reported in APPLIED PHYSICS LETTERS, 1966, Vol. 8, No. 5, Page 111 (S. M. Sze and G. Gibbons) and that the reverse withstanding voltage is lowered in a certain extent either in Si or GaAs.
On the other hand, in order to enhance performance of a microwave transistor, it is required to lower the base resistance. Therefore, it becomes necessary to select the doping amount at the base greater. As a result, the reverse withstanding voltage between the emitter and the base of the microwave bipolar transistor can taken only low value.
When an operation point of the transistor is selected at a deeper point, such as that of B-class bias and so forth in order to achieve high output and high efficiency with such microwave transistor similar to an amplifier circuit widely used in a low frequency band, an input signal of the microwave is applied between the base and the emitter in superimposition to a bias voltage to exceed the reverse withstanding voltage between the base and emitter by a partial microwave waveform.
When a current is flown in a reverse direction between the base and emitter of the bipolar transistor even for a substantially short period, hfe of the transistor may be degraded. This has been reported in IEEE, 1970, Vol. ED-17, No. 10, Page 871 (B. A. Mcdonald).
In order to avoid reverse current between the base and the emitter to be a cause of degradation of hfe, it may be a solution for connecting a P-N junction diode in reverse connection, between the base and the emitter, for example. This method has been reported in Electronics Magazine, December, 1971, Page 92.
In this method, the capacity of the base and emitter of the bipolar transistor becomes a sum of the own capacity of the transistor and the capacity of the diode to significantly degrade high frequency characteristics. Therefore, it is not applicable for the circuit handling the signal of the microwave.
Therefore, in the microwave band, it is typical that the transistor is used in single or the transistors are used in a simple parallel connection. In case of a common-emitter type circuit, the transistors are frequently used with A-class bias. Therefore, it is inevitable to lower the efficiency.
In the case where the B-class bias is forcedly used, the common-base connection is employed to lower impedance at the input side and thus to make the voltage lower even when the input signal is in reverse phase. This holds disadvantage in that a gain at one stage of amplifier is low.
It should be noted that an example of a manner of connection, in which P-N junction diode being reverse connection is connected in parallel between the base and emitter in the microwave band oscillator circuit, has been disclosed in Japanese Unexamined Patent Publication No. 59-27485. However, even in this case, the microwave signal is not supplied to the added diode per se. Instead, the diode is inserted in the bias circuit so as to restrict deepening of the bias point according to growth of the oscillation. Thus, in the disclosed art, attention has been paid so as not to degrade the high frequency characteristics.
However, in this method, the bias point is fixed to make it not applicable for the microwave amplifier circuit, in which is required to restrict the peak of the microwave signal. Accordingly, in the conventional microwave amplifier circuits including a push-pull type the bias point is selected at shallower points so that a sum of the bias voltage and the peak voltage of the microwave signal will not exceed the reverse withstanding voltage between the base and the emitter.
In this case, a large current flows through the transistor even when the microwave signal is not present to subject lowering of efficiency and restriction of output. On the other hand, when the bias voltage is forcedly deepen, the diode is added in parallel but in the reverse direction between the base and emitter. This results in a problem of degradation of high frequency characteristics.
In view of the problems as set forth above, the push-pull circuit is rarely employed in the microwave band. However, for example, in IEEE, Microwave and Millimeter Wave Monolithic IC Symposium, held 1986, an example of the push-pull type oscillator and so forth has been discussed in a literature issued by Allen F. Podell and William. W., Nelson, "High Volume, Low Cost, MMIC Receiver Front End".
Furthermore, in the reason set forth above, it is rare to employ the bipolar transistor in a frequency multiplier circuit of the microwave. Typically, a frequency multiplier circuit employing a varacter diode is used. Such frequency multiplier circuit employing the varacter diode is quite difficult to adjust and has quite large multiplying loss.
Therefore, in order to realize the microwave frequency multiplier circuit employing the bipolar transistor, it becomes necessary to reduce the impedance at the input side by employing the common-base connection so that the input voltage can be suppressed to be lower voltage even in the case where the input signal is in reverse phase. However, this hold disadvantage in low gain of the multiplier circuit.